Infrared cameras play an important role in the surveillance, law enforcement and military applications. Infrared cameras provide thermal images of objects in a thermal scene without illumination. Infrared cameras contain infrared (IR) sensors or focal plane array (FPA), which should be cooled to function properly. In some applications, infrared sensors are cooled to cryogenic temperature range. For these applications, a Stirling Cycle Rotary Cooling Engines are used to provide the cooling power.
Conventional Stirling engines generally have a compressor and an expander connected to a crank mechanism driven by an electrical motor. The compressor, also known as a pressure wave generator, is attached to the warm end of the expander and delivers acoustic power (compressor PV work) into the expander warm end inlet. Compressor PV work is the integration of the pressure-volume curve over one thermodynamic cycle or one complete revolution of the crank shaft. Compressor PV work has a unit of energy, and when derived over time, it is defined as acoustic power. The expander recovers this work at the cold end by causing the gas to expand and thus absorb heat from external power source such as an IR sensor. The gas expansion is achieved mechanically by placing the expander piston and compression piston at 90° mechanical phase to each other relative to the crank shaft. A working fluid, typically a noble gas, is compressed at the warm end and is expanded at the cold end. At the distal tip of the expander coldwell, when the expander piston is being pulled backward to iso-thermally expand the working gas, heat is absorbed from the load and very low temperatures are achieved due to efficient thermal isolation between the warm and cold end of the expander unit. Temperature can reach down to the cryogenic range, e.g., about 77° K. The IR sensor or FPA is attached to the coldwell to be cooled. A conventional Stirling engine is described in U.S. Pat. Nos. 7,555,908 and 7,587,896 and references cited therein, which are incorporated herein by reference in their entireties.
In the field, infrared cameras are often mounted on law enforcement and military vehicles, including helicopter, drones, naval vessels, military all-terrain vehicles, etc. These vehicles often experience shocks, such as recoils from a discharging weapon. For example, recoils from cannon fires can send damaging vibrations to equipments installed in the vicinity. These vibrations can damage the expander in the Stirling engine, and more specifically the weld between the cold finger and the Dewar body, which can cause the loss of vacuum within the Dewar. This load can also apply a bending stress on the cold finger, which can cause a displacement of the IR sensor or the FPA, which is mounted on the distal end of the cold finger. The vibrations can also break the wires that connect the IR sensor to the central processing unit.
Hence, there remains a need for rugged integrated IR detector and cooler assembly that can withstand damaging vibrations encountered in the field.